The present invention relates to a reflection-type (transflective) color liquid crystal display device, more specifically, to a structure of a reflection-type color liquid crystal display device containing a color filter therein, capable of a multicolor display.
A reflection-type liquid crystal display device for performing a monochrome display using a TN (twisted nematic) liquid crystal cell or an STN (super twisted nematic) liquid crystal cell, is mainly used as a conventional reflection-type liquid crystal display device. However, in recent years, there has been a growing demand to display colors and reflection-type color liquid crystal display devices containing color filters therein have been vigorously developed.
The reflection-type liquid crystal display devices containing color filters therein are broadly classified into the following three examples.
The first conventional example uses a liquid crystal mode without a polarizing film. There is a guest-host LCD in which a black dye is mixed in a liquid crystal material, a polymer-dispersion LCD in which a liquid crystal material is dispersed in a polymer and so on. Since none of them uses a polarizing film, they are excellent in brightness but low in contrast, thus they have not been realized for practical use yet.
The second conventional example is a reflection-type liquid crystal display device using one polarizing film and containing a reflector inside a liquid crystal display device. Moreover, this example is divided into two types. One type uses a self-contained reflector of a mirror surface and has a diffusing layer provided on the surface thereof, and the other uses a reflector with scattering properties. Since both types use only one polarizing film, they are also excellent in brightness but low in contrast.
In the type using a self-contained reflector of a mirror surface, though it is bright in a direction of regular reflection of incident light, it becomes abruptly darker at other angles, that is, the viewing angle characteristic thereof is quite poor. In the other type using a reflector with scattering properties, it is difficult to control scattering properties and the fabricating process becomes complicated.
The third conventional example is a liquid crystal display device using two polarizing films and having a color filter provided in a typical monochrome liquid crystal display device. Since this example uses two polarizing films, it is excellent in contrast, but it has a disadvantage of a dark display. However, a reflection-type polarizing film, which has a transmission axis and a reflection axis to reflect the light linearly polarized in a direction of the reflection axis, is used for a lower polarizing film, thereby brightness is improved and this reflection-type liquid crystal display device is considered for practical use. The conventional example using a reflection-type polarizing film is disclosed in, for example, Japan Patent Laid-open No. Hei 10-3078. 
However, in this Patent Bulletin, disclosed is a TN-mode reflection-type display device mainly using active matrix (drive) elements. A liquid crystal display device using active matrix (drive) elements is excellent in contrast but the brightness thereof is lowered due to lowering of an aperture ratio. In contrast to the above, a passive matrix liquid crystal display device without active matrix (drive) elements is superior in brightness because of its higher aperture ratio.
Moreover, it is disclosed that a reflection-type color liquid crystal display device can be provided by using an STN liquid crystal cell having a twist angle of 90xc2x0 or more, a retardation film and a reflection-type polarizing film in the aforesaid Patent Bulletin. However, the liquid crystal display device described in the embodiments is a typical monochrome display STN liquid crystal display device provided with a color filter.
A typical monochrome display STN liquid crystal display device is a normally-white-type for obtaining a white display during no applied voltage. For enhancing whiteness more than brightness, the transmittance on a long wavelength side is suppressed, thus the average transmittance is lowered. Moreover, a black display of ON state is slightly bluish black, therefore the performance as a shutter is not excellent.
In other words, the reflection-type color liquid crystal display device that is a typical monochrome display STN liquid crystal display device provided with a color filter, performs a dark display due to lowering of brightness, more than that, since the shutter performance varies from color to color, excellent colors can not be obtained.
The present invention is made to solve the aforesaid disadvantages, and its object is to provide a reflection-type color liquid crystal display device with a bright display, high contrast and excellent colors.
In order to achieve the above object, the reflection-type color liquid crystal display device according to the present invention comprises: an STN liquid crystal cell having nematic liquid crystal, which is aligned at a twist angle of 180xc2x0 to 270xc2x0, sandwiched between a transparent first substrate having first electrodes and a transparent second substrate having second electrodes, in which the first electrodes and the second electrodes opposing to each other across the nematic liquid crystal, and the first substrate or the second substrate is provided with a color filter of a plurality of colors; an absorption-type polarizing film placed on the visible side of the second substrate and having a transmission axis and an absorption axis; a retardation film placed between the absorption-type polarizing film and the second substrate; and a diffusing layer, a reflection-type polarizing film having a transmission axis and a reflection axis, and a light absorbing layer absorbing almost all incident light, which are arranged in order on the other side of the visible side of the first substrate.
Moreover, by using the so-called Z-type retardation film which satisfies conditions of nx greater than nz greater than ny, where nx is the refractive index in a stretching direction of the retardation film, ny is the refractive index in a direction orthogonal to the stretching direction, and nz is the refractive index in a thickness direction, the viewing angle is improved and incident light from the surroundings is effectively utilized, thus a bright display can be obtained.
Alternatively, a typical retardation film which satisfies conditions of nx greater than ny=nz, where nx is the refractive index in a stretching direction of the retardation film, ny is the refractive index in a direction orthogonal to the stretching direction, and nz is the refractive index in a thickness direction, may be used.
Furthermore, in the above either case, it is preferable that a value of a difference xcex94R=Rsxe2x88x92Rf ranges from 0.27 xcexcm to 0.35 xcexcm, where Rs is a xcex94nd which is the product of a difference xcex94n in the birefringence of nematic liquid crystal of the STN liquid crystal cell and a cell gap d, and Rf is a retardation value of the retardation film, thereby the average transmittance of white becomes higher and an excellent black characteristic can also be obtained.
Even if a twisted retardation film is used in place of the aforesaid retardation film in the reflection-type color liquid crystal display device, a white characteristic with high transmittance, an excellent black characteristic, and a wide viewing angle characteristic can be obtained.
In the above case, it is preferable that a value of a difference xcex94T=|Ts|xe2x88x92|Tc| in absolute value of each twist angle ranges from 10xc2x0 to 30xc2x0 and a value of xcex94R=Rsxe2x88x92Rc ranges from 0.15 xcexcm to 0.30 xcexcm, where Rs is a xcex94nd which is the product of a difference xcex94n in the birefringence of nematic liquid crystal of the STN liquid crystal cell and a cell gap d, Ts is a twist angle of the STN liquid crystal cell, Rc is a xcex94nd value of the twisted retardation film, and Tc is a twist angle of the twisted retardation film.
Additionally, it is preferable that the color filter has a maximum transmittance of 80% or more and a minimum transmittance of 20% to 50% due to its spectral characteristics.
The thickness of the first substrate of the STN liquid crystal cell is preferably thinner than that of the second substrate.
Furthermore, in place of the light absorbing layer, a translucent-type light absorbing layer through which part of incident light passes, and a backlight may be arranged outside the reflection-type polarizing film in the aforesaid reflection-type color liquid crystal display device. Thereby, a bright color display can be obtained even in a dark circumstance such as at night by lighting of the backlight.
Also in this case, it is preferable to satisfy conditions of nx greater than nz greater than ny, where nx is the refractive index in a stretching direction of the retardation film in use, ny is the refractive index in a direction orthogonal to the stretching direction, and nz is the refractive index in a thickness direction.
Alternatively, a twisted retardation film may be used in place of the retardation film.
The translucent-type light absorbing layer is preferably a plastic film dyed with a black dye or a black pigment to have a transmittance of 20% to 60%.
Alternatively, the translucent-type light absorbing layer may be formed by printing with black ink to have a transmittance of 20% to 60% on the rear face of the reflection-type polarizing film or on the front face of the backlight.